In wireless communication systems in particular, communication quality and communication capacity often have an inverse relationship. For example, as communication capacity is increased, such as through more dense reuse of traffic channels, signal quality may be decreased, such as through each such traffic channel experiencing higher levels of interference energy. Accordingly, wireless communication service providers must often balance providing desired levels of communication capacity with service quality issues.
In code division multiple access (CDMA) networks, for example, a number of communication signals are allowed to operate over the same frequency band simultaneously. Each communication unit is assigned a distinct, pseudo-random, chip code which identifies signals associated with the communication unit. The communication units use this chip code to pseudo-randomly spread their transmitted signal over the allotted frequency band. Accordingly, signals may be communicated from each such unit over the same frequency band and a receiver may despread a desired signal associated with a particular communication unit. However, despreading of the desired communication unit's signal results in the receiver not only receiving the energy of this desired signal, but also a portion of the energies of other communication units operating over the same frequency band. Accordingly, CDMA networks are interference limited, i.e., the number of communication units using the same frequency band, while maintaining an acceptable signal quality, is determined by the total energy level within the frequency band at the receiver.
It is therefore desirable to control the amount of energy radiated within a particular service area to thereby reduce interfering energy experienced by subscriber units operating therein. For example, in the aforementioned CDMA networks, transmitted signals are often power controlled to reduce energy transmitted within the CDMA frequency band while maintaining sufficient power to provide an acceptable signal at a receiving unit. Through intelligent power control, excess energy within the service area may be limited and, therefore, signal quality improved and/or capacity increased.
Further capacity and/or signal quality improvement may be provided in communication systems through the use of directional antenna beams in the communication links. For example, adaptive array antennas may be utilized to provide enhanced signal quality through advanced “beam forming” techniques as shown and described in the above referenced patent application entitled “Practical Space-Time Radio Method for CDMA Communication Capacity Enhancement.” For example, angle of arrival (AOA) information determined from a received signal at an adaptive array antenna may be utilized in accurately determining beam forming coefficients for use in providing narrow beams the reverse link in order to provide improved capacity.
However, the use of such narrow beams in providing communications links, although generally effective in containing the area in which signal energy is radiated or from which radiated energy is accepted, introduces unique problems associated with their implementation. For example, cellular or personal communication services (PCS) systems using CDMA communication techniques often utilize both a pilot signal and a traffic signal to establish communications. The pilot signal generally provides a known signal and is used by receiving devices in demodulating a traffic signal. In the forward link, i.e, the base station to subscriber user link, a common pilot signal is typically used for multiple subscriber units, such as all subscriber units in a cell or a sector. Accordingly, it is typically desirable to provide this pilot signal throughout an area in which multiple subscriber units are likely to be located.
The use of narrow beams for reducing radiated and/or accepted energy as discussed above can be problematic with respect to use of such a pilot signal. For example, if the pilot signal were to be transmitted in a narrow beam corresponding to the traffic signal of a particular subscriber unit, other ones of the plurality of subscriber units may not receive the pilot signal for use in demodulating their corresponding traffic signal. Accordingly, it is often desirable to provide the pilot signal in an area larger than that of the narrow beam associated with a particular subscriber unit. However, this often results in a phase mismatch problem at one or more of the subscriber units. Specifically, as the link channel associated with the pilot signal (e.g. wide beam) is not the same as that of the link channel associated with the traffic signal (e.g. narrow beam), the phase information extracted from the pilot signal may no longer accurately correlate with the traffic signal as received by a subscriber unit. Although perhaps providing phase matching to within acceptable limits for lower order modulation schemes, such as BPSK, such a phase mismatch is likely to result in unacceptable communication errors, such as an excessive bit error rate (BER), in higher order modulation schemes, such as QPSK, 8PSK, etcetera.
Accordingly, a need exists in the art for systems and methods which provide for the use of optimized beams, such as beams having a minimized or otherwise reduced beam width, to thereby control the amount of interference energy radiated and/or accepted, while providing a desired and predictable signal phase relationship. A further need exists in the art for such systems and methods to provide the phase relationship relative to a pilot signal associated with an antenna beam having a size and/or shape substantially different than that of a corresponding traffic signal communicated in an optimized beam configuration.